A photovoltaic (PV) power source, such as a solar cell array, may be used to drive an electric load, such as an electric motor. While photovoltaic cells are DC power sources, such a motor driven by the source may be either AC or DC. The use of solar panels, or solar cell arrays, to drive AC electric motors for water pumps or electrical generators, or for providing DC power directly to a battery, are common for off-grid locations, such as off-grid development projects, eco-friendly living and working spaces, and for desalinizing water, especially in remote locations.
Most electronic devices may be described by some parameters that stay constant within certain operating environments that cover almost the entire operating range of the device. This is not the case for solar cells. Most solar cells are described by their current, voltage, or power output at standard operating conditions, with an understanding that they will spend a large portion of their operation outside those conditions. This is because the amount of electrical power generated by a PV system depends on solar irradiance (or solar energy per unit area of the solar cell array surface) and may be affected by a variety of other factors, such as temperature. Further, the irradiance may suddenly change due to, for example, cloud cover.
Power can be stated as P=IV, where P is power, I is current and V is voltage. Accordingly, the possible output of a solar cell array in any given environment may be shown as an I-V curve to show the range of available voltages and corresponding current values if the system were to be operated at those voltages. The point of the curve at which P=IV is maximized, or the knee of the curve, represents that maximum power point, or MPP, at which the power source may be operated.
FIG. 1 shows I-V curves at different irradiance values. As shown, changes in the environment, such as occlusion of solar panels or otherwise lowering of available irradiance, can shift the I-V curve, replacing it with a different curve. Such a change typically results in a different voltage level that corresponds to the new MPP. Accordingly, an MPP tracking, or MPPT, system will attempt to determine the new maximum power point of the curve.
Accordingly, a number of parameters of both the solar cell array and the electric load are selected and tracked as part of an MPPT system in order to use available PV resources as efficiently and completely as possible. In such a system, both the power source and the electric motor may be exposed to conditions, such as environmental conditions, that require constant adjustments to both the power source and the load in order to maintain an efficient implementation of the system.
PV power systems are therefore typically paired with a control system to implement maximum power point tracking (MPPT) for the system. The location along the I-V curve at which the power source is operated may be determined by a load applied to the PV source, so that increasing a resistance in the load by, for example, modifying the frequency of a motor, may in turn increase the voltage output by the power source. A corresponding change in current will then keep the power output along the I-V curve.
MPPT systems are typically complex, require a wide array of sensors, and require substantial processing powers to operate. Further, MPPT systems may be slow due to their iterative nature, and typically require the comparison of multiple calculated metrics, further increasing the amount of processing required and may require additional time to calculate. Such time delays may detract from power that would otherwise be available to power the load, and such complexity may lead to expensive repairs and troubleshooting, as well as difficulty in implementation.
Further, many existing MPPT systems may be unable to discern between local maxima and global maxima for power, may oscillate around an MPP, or may converge inefficiently.
There is a need for a simple MPPT system and method that allow for accurate tracking of the maximum power point of a PV system without substantial processing or power overhead, and with a minimal number of discrete calculations.